In vitro assays for a murine megakaryocytic progenitor cell (CFU-M) have increased the understanding of the regulation of platelet production. These studies have suggested the relative independence of CFU-M to acute alterations in the platelet count, and the prominent role of the megakaryocyte in response to such changes. We propose to study further the regulation of murine megakaryocytopoiesis in human diseases. Regulation studies in vivo will focus on the role of the immune system in the control of early megakaryocytopoiesis. Congenitally athymic mice will be employed to examine the influence of T cells on CFU-M. In vitro studies will focus on the purification of the megakaryocyte colony stimulating activity (meg-CSA), a requirement for the growth of megakaryocyte colonies in culture. A cell line producing meg-CSA will be used as the source of material for purification studies. The regulation of endomitosis will be studied by examining the effects of experimental alterations of the platelet count on the CNA content of cultured megakaryocytes. Measurements of ploidy will be performed using DNA binding fluorescent dyes. Finally, methodologic improvements for the identification of human colonies will be undertaken using immunoperoxidase techniques. An assay for human CFU-M will be used to analyze abnormalities of regulation or of progenitor cells in thrombocytopenic and thrombocytotic states.